Spamsnake connection timeouts

This one has me stumped, and I'd like to be positive it's not on my end before taking it further. I've had the Spamsnake running successfully for about 8 months, but just two weeks ago a handful of people have been getting "connection timed out" errors when trying to send us email. This also happens when trying to telnet to our port 25. However, these people can access our web page on port 80 just fine.

I haven't made any configuration changes, and there is nothing showing up in either the firewall or spamsnake logs. I did turn off greylisting after the problem cropped up. It seems like an ISP problem, but the people are pretty spread out geographically. I've added the domains to the sender access list, but since these people aren't even showing in the logs, I don't think their email is making it that far. Is there anything in the spamsnake that could reject on port 25 without any log notification? Can you guys think of anything else I can do to troubleshoot on my end?